The present invention relates to optical systems, and more particularly, to a zoom lens system adapted for use in a compact camera that generates still images with an electronic image sensor.
Digital still cameras are rapidly gaining in popularity with consumers. They permit high resolution color images to be generated for subsequent printing by conventional ink jet or laser printers. The quality of the prints approaches that of conventional silver halide photographic film prints. More importantly, the digital images can be stored on different forms of media such as flash memory cards, floppy disks and compact disks, manipulated and stored in a personal computer, inserted into computer graphics applications such as word processing programs, sent to friends and relatives via e-mail, and transmitted as part of web pages. From an environmental standpoint, digital still cameras are attractive because they eliminate the need for chemical developer solutions.
A digital still camera includes a electronic image sensor which typically comprises an array of charge coupled detectors (CCDs). In the past, fixed focal length lens systems have been used with CCDs. For many applications, such systems are inadequate because a fixed focal length system cannot simultaneously provide a large angle of view and sufficient resolution to allow detailed examination of specific parts of the field of view. Although electronic zooming can be performed, i.e. zooming wherein a portion of the field of view is selected and electronically magnified to fill the entire viewing screen, such zooming does not increase the resolution of the original image captured by the CCD. Alternatively, the resolution of the original image can be improved by increasing the number of CCD pixels, but this approach significantly adds to the cost of the device and diffraction effects limit the extent to which pixel size can be reduced.
Thus, there exits a continuing need for improved variable power lens systems for use with electronic imaging systems. As is known in the art, variable power lens systems can have a discrete number of focal lengths at which the image is in focus at a fixed location, e.g. a dual power system, or can have a focal length which can vary continuously while producing a focused image at a fixed location, e.g. a zoom lens system. Alternatively, the image location need not be kept fixed, in which case, either the detecting device (CCD) or the lens system can be moved as the focal length of the lens system is varied either discretely or continuously.
U.S. Pat. No. 5,745,301 of Ellis L. Betensky et al. entitled VARIABLE POWER LENS SYSTEM FOR PRODUCING SMALL IMAGES granted Apr. 28, 1998 discloses variable power lens systems for use with electronic imaging systems, e.g. systems employing CCDs.
In developing a zoom lens system for a digital still camera, the following principles of conventional lens design must be taken into consideration:
1) The manufacturing cost of a lens element is primarily determined by the volume of the element and its surface area. For this reason, in most commercial applications lens designers typically try to minimize lens element diameters and thicknesses relative to the image size.
2) In zoom lens design, one or both of the pupils typically moves during zooming. This means that lens diameters must be increased to transmit the entire imaging bundle throughout the zooming range. To minimize this increase in lens diameters, lens designers typically use lens elements of relatively large powers. This, in turn, results in increased aberration contributions because of the large powers, and necessitates the inclusion of more lens elements for aberration correction.
Many hand held video cameras have zoom lens systems, however, their lenses are much larger with respect to the image sensor size than would be acceptable in a compact digital camera for taking still pictures. Furthermore, the need for a distantly located exit pupil in order to satisfy the color filter requirements of the electronic image sensor exacerbates the problem of size minimization. An additional complication involved in providing a zoom lens system in a compact digital still camera relates to the placement of the iris diaphragm. Ideally, the diaphragm should move independently during zooming and focusing to minimize aberration changes caused by the internal change of magnification for both object and pupil during zooming. But this usually requires additional mechanical linkages and controls. At the other end of the extreme is the least complex design in which the iris diaphragm remains fixed in position and opening size both during zooming and focusing.
Another major problem in designing lens systems for compact cameras is that large lens powers are needed to assure both small lens diameters and small lens system lengths. Large lens powers introduce large amounts of undesirable aberrations, which usually requires complex individual lens element groups to correct for these aberrations. This in turn undesirably increases the size of the lens system as more physical space is required to accommodate the extra lens elements.